HIGH EFFICIENT FULLY ION-IMPLANTED, CO-ANNEALED AND LASER-STRUCTURED BACK JUNCTION BACK CONTACTED SOLAR CELLS

摘要

For achieving high energy conversion efficiencies in back junction back contacted (BJBC) solar cells, it is important to form excellent pn (high-low) junctions between emitter (back surface field (BSF)) and the base while maintaining high bulk lifetimes, and to apply a rear-side passivation scheme which passivates well both polarities - emitter and BSF - as well as the pn-junction meander between these regions. The latter aspect is of particular importance for small BSF indices (i.e., for a large pn-junction meander length), which are required to minimize lateral transport losses of majority carriers in the Si bulk. In this work, we demonstrate that ion-implanted, co-annealed junctions meet these requirements when SiO_2 - grown in-situ during the annealing process - is utilized as rear-side passivation. For sheet resistances of 130Ω/□ (60Ω/□) recombination current densities down to 29 fA/cm~2 (40 fA/cm~2) are achieved for the boron emitter (for the phosphorus BSF) regions with this passivation scheme. By contrast, Al_2O_3 yields a slightly better passivation quality on the boron doped emitter, but the much worse passivation quality of Al_2O_3 on the phosphorus doped BSF regions and, in particular, on the pn-junction meander between emitter and BSF, strongly overcompensates this benefit. Therefore, ion-implantation and co-annealing including in-situ oxide growth does not only strongly reduces process complexity (only one high temperature step required), but is also preferable regarding the efficiency potential. This is demonstrated on BJBC cells with average conversion efficiencies ＞23.5% and a champion efficiency of 24.1% (in-house measurement, designated area of 3.96cm~2). Under reserve of the ongoing independent confirmation, this would correspond to the highest efficiency reported so far for fully ion-implanted Si solar cells. We achieve these results with an optimized laser structuring that realizes BSF indices down to 450μm.